drm/exynos: fix cancel page flip code

drm/exynos: fix cancel page flip code

Driver code did not remove event from the list of pending events before destroy.
As a result drm core later tried to inspect invalid memory location.
The patch replaces removal code with call to core helper.

The bug was detected using KASAN:

[   10.107249] ==================================================================
[   10.107518] BUG: KASAN: use-after-free in drm_release+0xe9c/0x1000 at addr ffffffc089154a18
[   10.107784] Read of size 8 by task modetest/103
[   10.107931] =============================================================================
[   10.113191] BUG kmalloc-128 (Not tainted): kasan: bad access detected
[   10.119608] -----------------------------------------------------------------------------
[   10.119608]
[   10.129243] Disabling lock debugging due to kernel taint
[   10.134551] INFO: Allocated in drm_mode_page_flip_ioctl+0x500/0xa98 age=4 cpu=0 pid=103
[   10.142532] 	alloc_debug_processing+0x18c/0x198
[   10.147043] 	___slab_alloc.constprop.28+0x360/0x380
[   10.151906] 	__slab_alloc.isra.25.constprop.27+0x54/0xa0
[   10.157197] 	kmem_cache_alloc_trace+0x370/0x3b0
[   10.161709] 	drm_mode_page_flip_ioctl+0x500/0xa98
[   10.166400] 	drm_ioctl+0x4c4/0xb68
[   10.169787] 	do_vfs_ioctl+0x16c/0xeb8
[   10.173429] 	SyS_ioctl+0x8c/0xa0
[   10.176642] 	el0_svc_naked+0x24/0x28
[   10.180204] INFO: Freed in exynos_drm_crtc_cancel_page_flip+0xe0/0x160 age=0 cpu=0 pid=103
[   10.188447] 	free_debug_processing+0x174/0x388
[   10.192871] 	__slab_free+0x2e8/0x438
[   10.196431] 	kfree+0x350/0x360
[   10.199469] 	exynos_drm_crtc_cancel_page_flip+0xe0/0x160
[   10.204762] 	exynos_drm_preclose+0x58/0xa0
[   10.208844] 	drm_release+0x1f0/0x1000
[   10.212491] 	__fput+0x1c4/0x5b8
[   10.215613] 	____fput+0xc/0x18
[   10.218654] 	task_work_run+0x130/0x198
[   10.222385] 	do_exit+0x700/0x2278
[   10.225681] 	do_group_exit+0xe4/0x2c8
[   10.229327] 	SyS_exit_group+0x1c/0x20
[   10.232973] 	el0_svc_naked+0x24/0x28
[   10.236532] INFO: Slab 0xffffffbdc2a45500 objects=32 used=10 fp=0xffffffc089154a00 flags=0x4080
[   10.245210] INFO: Object 0xffffffc089154a00 @offset=2560 fp=0xffffffc089157600
[   10.245210]
...
[   10.384532] CPU: 0 PID: 103 Comm: modetest Tainted: G    B           4.5.0-rc3-00748-gd5e2881 #271
[   10.398325] Call trace:
[   10.400764] [<ffffffc000091428>] dump_backtrace+0x0/0x328
[   10.406141] [<ffffffc000091764>] show_stack+0x14/0x20
[   10.411176] [<ffffffc00089c550>] dump_stack+0xb0/0xe8
[   10.416210] [<ffffffc000395778>] print_trailer+0xf8/0x160
[   10.421592] [<ffffffc00039b5cc>] object_err+0x3c/0x50
[   10.426626] [<ffffffc00039d630>] kasan_report_error+0x248/0x550
[   10.432527] [<ffffffc00039da50>] __asan_report_load8_noabort+0x40/0x48
[   10.439039] [<ffffffc000b5b724>] drm_release+0xe9c/0x1000
[   10.444419] [<ffffffc0003d340c>] __fput+0x1c4/0x5b8
[   10.449280] [<ffffffc0003d3884>] ____fput+0xc/0x18
[   10.454055] [<ffffffc000101aa8>] task_work_run+0x130/0x198
[   10.459522] [<ffffffc0000bc058>] do_exit+0x700/0x2278
[   10.464557] [<ffffffc0000bdcfc>] do_group_exit+0xe4/0x2c8
[   10.469939] [<ffffffc0000bdefc>] SyS_exit_group+0x1c/0x20
[   10.475320] [<ffffffc000087530>] el0_svc_naked+0x24/0x28

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>